Some of the conventional undercut thermoplastic lids, and in particular lids destined to act, first as an over-lid covering over the lid sheet sealed on the container, and then as a permanent lid after the destruction of the lid sheet, are prefabricated by thermoforming in a thermoplastic band, with a mold of suitable shape, and are thereafter cut from the thermoplastic band before being fixed on the corresponding containers by screwing or snap-fitting.
The dimensions of these lids vary from one batch of lids to another. Variations in the dimensions may be due to the use of different basic materials having different degrees of shrinkage. Furthermore, the dimensions of the edges of the thermoplastic containers are also prone to variations. It has been found that wear of the equipment used for producing the lids and the containers and for cutting them, also contributes to such variations in the dimensions. It has also proved difficult to correctly center the prefabricated lids with respect to the edge of the containers.